1 Field of the Invention
This invention relates to a technique for increasing the output power from a flowing gas laser system, in particular for minimizing conditions which enhance glow-to-arc phenomena by removing undesired gas constituents which reside in a boundary layer which envelops the electrical sheath region along the electrode surfaces of a laser.
2 Description of the Prior Art
A flowing gas laser system, frequently referred to as a convection gas cooled laser, is designed such that the flowing medium passes through an electrode configuration, thereby carrying away the heat which is generated due to electrical discharge in the laser gas. The average optical power output of such a laser system is determined by the product of the energy per optical output pulse and the frequency of the optical output pulses. The average power output from the laser system may be significantly increased by maximizing the optical pulse repetition rate.
Attempts to increase the average power output of flowing gas laser systems have met with limited success. Experimental studies indicate that as the pulse repetition rate is increased, a threshold repetition rate is reached at which there will be a glow-to-arc transition, thereby limiting the maximum attainable pulse repetition rate. There are at least two primary factors which limit the maximum pulse repetition frequency of a given laser with a specific flow velocity through the cavity: (1) the optical homogeneity of the gases in the laser cavity, (2) the arcing properties of the gases in the cavity.
Optical homogeneity must be established in the laser cavity between succeeding laser pulses if it is desirous to obtain good beam quality, i.e., uniform, high power density. To obtain this uniform optical homogeneity in the cavity following a given laser pulse, sufficient time must be provided so that acoustical disturbances due to previous discharges and other disturbances will subside before the application of the succeeding pulse; therefore, the required clearing time for achieving or restoring optical homogeneity is an important factor in determining the maximum pulse repetition of a laser.
A second important factor is the time required to clear the residual gases in the aerodynamic boundary layer on the electrodes. Generally, these gases will be more prone to cause arcing rather than the desired glow discharge because of residual effects from the previous pulse, higher gas temperature, gas decomposition, and electron attachment. An arc may start almost immediately on the application of high voltage to the electrodes, or may result from a delayed glow-to-arc transition. The severity of the problem will determine the length of the glow period prior to the transition to an arc.
Attempts to minimize arcing conditions have included reducing localized regions of electric field enhancement at the electrode surfaces. Such regions alter the electric field established by the electrodes, thereby enhancing arcing conditions. Although such imperfections on the electrode surface have been removed, the residual effects from previous pulses still remain so as to limit the pulse repetition frequency.